Multilayer printed circuit boards comprise a stack of individual printed circuit boards or innerlayers separated by dielectrical material. The circuitry of the several innerlayers is electrically connected by bored and plated- through holes. Multilayer printed circuit boards provide circuitry in a three-dimensional array and are therefore advantageously space-saving, relative to individual printed circuit boards, which provide at most two layers of circuitry on a two-sided board.
These printed circuit boards are commonly provided with internal ground and power planes. These internal planes are frequently solid sheets of copper interrupted only by clearance holes (the perforations required for electrically isolating the through hole pattern of the printed circuit board) Ground and power planes provide power voltage and current and ground connections for the components of the multilayer printed circuit. A second function of the ground and power planes is to provide electromagnetic shielding for the multilayer printed circuit board and reduce the electromagnetic and radio frequency interference. Multiple ground and power planes and additional ground planes on the surface layers with the conductive pattern are common.
When components are mounted on a multilayer printed circuit board and mass soldered in place at temperatures on the order of about 275.degree. C. (527.degree. F.), a severe thermal shock is applied to the insulating layers placed between two copper planes, such as the insulating layer between an internal ground plate and ground shield on the surface surrounding the conductor pattern. Frequently, delamination will occur and blisters will form between the ground shield on the surface and the internal ground or power plane. Delamination and blistering have been a problem with multilayers made by a fully additive, semi-additive, or subtractive sequential processes.
In a conventional subtractive procedure for forming multilayer printed circuit boards, the innerlayers are formed in the manner of individual two-sided printed circuit boards. A blank board formed of epoxy that is covered on each side with a metal layer, such as copper, is coated on each side with a layer of primary photoresist. The primary photoresist may be coated as a liquid resist or applied as a dry film. The layers of photoresist are exposed to patterned actinic radiation by placing artwork over the photoresist layers and directing light through the artwork to the photoresist layers. Subsequently, the exposed photoresist layers are developed in an appropriate aqueous medium to remove exposed or unexposed portions of the photoresist layers, depending upon whether the photoresist is positive-acting or negative-acting. The metal layer is then etched away in the areas from which the overlying photoresist has been removed. After etching, the remaining portions of the photoresist layers are stripped from the surfaces of the board. In the conventional procedure, the copper surfaces which have been uncovered by the stripping process are then treated to enhance adhesion of the copper to the pre-preg in subsequent steps.
The individual innerlayers are stacked with alternating layers of "prepreg" material, which typically comprises an epoxy/fiberglass composition. The assembly of innerlayers and prepregs is then pressed together and heated. The prepreg material initially flows under the heat and pressure, filling the voids between the innerlayers, and eventually cures under these conditions to form a hard permanent dielectric material separating the individual circuitry layers.
It has been the practice to apply an initial, strongly adherent oxide layer to the copper to enhance the bond between the copper conductive patterns and the insulating layers. The oxide layers are usually applied by immersing the copper surface in hot (40.degree.-110.degree. C.), strongly alkaline, hypochlorite solutions. This immersion produces a black, dendritic oxide layer with a high surface area for adhering to organic films, coatings, and laminated layers. In the printed circuit industry, this oxide is commonly called "black oxide".
To electrically connect the circuitry of the several innerlayers, holes are drilled through the cured assembly. The heat so generated causes an epoxy resin "drill smear" on the inner layer copper surface around the drilled hole prior to the electroless plating of the hole wall. The hole walls are plated to create electrical connections to the inner copper planes. The desmearing and plating solutions dissolve the black oxide surrounding the holes and leave non-adherent rings around the holes. This is known as "pink ring" because of the ring of copper visible in the pattern of black oxide. At the pink ring, the adhesion between the copper plane and the laminated insulating layer over it is reduced. Ionic contamination and the failure of insulation between holes occur where pink ring is found.
As can be seen, the above process requires a number of steps, and it is a primary object of the present invention to produce multilayer printed circuit boards utilizing fewer production steps.
It is a related object of the invention to eliminate the need for an oxide treatment of the copper layers to improve their adhesion to the epoxy/fiberglass substrate.
It is a further related object of this invention to eliminate pink ring on the innerlayers of a multilayer printed circuit board.
It is yet another related object of this invention to eliminate wedge voids caused by the action of desmearing chemicals at the black oxide/epoxy interface on the innerlayers.
It is another object of this invention to provide a multilayered printed circuit board in which the prepreg dielectric material is substantially replaced by a permanent innerlayer of photoresist material.
It is another object of this invention to provide permanent innerlayer resists which are very thin and can be imaged on contact.
It is a related object to provide permanent innerlayer resists having enhanced printing resolution capabilities.
It is a further related object to provide multilayer printed circuit boards having improved resolution of the circuitry images thereon.
Other objects of the invention include reducing the materials used in the process of producing multilayer circuit boards and limiting the negative impact on the environment associated with prior compositions and methods for making such boards.